Synthesis, structure elucidation, antioxidant properties, and theoretical calculations of new Schiff bases–isatin derivatives

Abstract

Isatin-derived Schiff bases are the subject of many studies, finding wide application areas in polymer technology, pharmaceutical industry, and medicine. In this study, a series of new Schiff bases were prepared from monothiocarbohydrazones based on isatins with different substituents (5-F, 5-Br, 5-I, and 5-MeO). The chemical structures of the synthesized compounds were determined using 1H NMR, 13C NMR, FTIR spectroscopic techniques, and elemental analysis. Antioxidant activity determinations of 23 compounds were performed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical quenching method. The highest percent inhibition value at 10 µM concentration was shown by compound number 22, 5-bromoisatin Schiff base containing 3-methoxy-4-hydroxy group. Compound 17, a 5-iodoisatin Schiff base containing 3-methoxy-4-hydroxy group, showed the highest antioxidant activity with an IC50 value of 9.76 ± 0.03 µM. In addition to the theoretical analysis of the compounds, both their spectroscopic and antioxidant properties were investigated. The ground-state geometries and some chemical reactivity parameters of the compounds were calculated using the B3LYP/6-311 + + G(2d,2p) approach. Besides intramolecular interactions, substituent effects, and some QTAIM parameters, the calculations were also performed to study the electronic properties of reactive N/O–H bonds and were used to interpret the experimental results. The effects of the electronic parameters and intramolecular interactions of reactive N/O–H bonds on the antioxidant properties of the compounds were investigated. Additionally, the relationships of DPPH reactions with delocalization indices of N/O-H bonds and the pattern of SET/HAT mechanisms with electronic variables were analyzed. Examination of electron and hydrogen atom transfer mechanisms has shown the dominance of electron transfer, supported by the correlation coefficients between IC50 values and SET reaction energies.